Gas-assisted injection molding is generally the preferred method for the production of large and complicated parts without sink marks and the like in thick rib sections or complicated cross-sectional areas. In gas-assisted injection molding these large and complicated parts are made of a hollow construction by injecting a gas into the interior of a part while it is being formed in a mold cavity. It is known that the gas injected into the interior of the mold cavity will follow the so-called "path of least resistance." As the molten plastic cools and hardens from the outside inwardly, the gas will penetrate the inner and thicker portions of the part which are generally softer and warmer. Thus hollow cavities are generally formed in the thicker sections where, for example, structural rib and the like may be located. The pressurized gas also expands these hollow cavities thereby forcing the molten plastic outwardly to fill the mold cavity. If the gas pressure is maintained at a relatively high level while the plastic part cools, the surface finish of the plastic part will be greatly enhanced by the minimization or elimination of sink marks, depressions, or like imperfections. Such hollow cavities can also result in considerable weight savings in the finished plastic part.
It is necessary to vent the pressurized gas in the part in order to relieve the pressure within the part before the mold can be opened. Many solutions have been tried to provide a satisfactory way for introducing the gas into the interior of the plastic part and then venting the gas before opening the mold cavity. Numerous apparatuses have been advanced in the gas-assisted injection molding art to allow injection of both molten plastic and a pressurized gas with subsequent venting of the pressurized gas. Generally, these approaches involve very complicated nozzles and/or sprue bushings which have a significant tendency to fail during operation. Operation of gas-assisted injection molding systems using these prior art systems generally require significant time and resources devoted to maintenance programs.
One such apparatus for venting the gas before opening the mold cavity is described in U.S. Pat. No. 4,943,407 (Jul. 24, 1990). Generally this patent discloses a method for venting the gas to atmosphere through a specialized sprue bushing in which the sprue bushing includes a first body part in which a second body part or cylindrical part is mounted for movement upon the activation of an actuator. The part has two flowpaths which are alternately used. One flowpath allows the passage of plastic through the sprue bushing into the sprue. The second flowpath allows injection or venting of gas from the interior of the associated mold cavity. However, mold sprues are generally small, and the moving parts thereof which allow the injection of plastic and gas alternately have proven to be difficult to operate due, in significant part, to the tendency of the flowpaths to become blocked with plastic material. In addition, such specialized sprue valves add significantly to the cost of the molds.
U.S. Pat. Nos. 4,935,191 (Jun. 19, 1990), 5,015,166 (May 14, 1991), and 5,066,214 (Nov. 19, 1991) discloses a complicated design employing a plastic shut-off valve within the nozzle and gas injection and gas venting through the sprue bushing. The gas venting passageway in the sprue bushing is prone to being blocked with plastic material during the plastic injection cycle as well as during the venting cycle. And the gas injection passageway is prone to being blocked during the venting cycle. Removal of such blockages would normally require removing the sprue bushing form the mold body and cleaning out the various small diameter gas passageways.
U.S. Pat. Nos. 4,781,554 (Nov. 1, 1988), 4,905,901 (Mar. 6, 1990), and 4,944,960 (Jul. 31, 1990) disclose complicated nozzle systems wherein both resin and pressurized gas are injected through the nozzle. Two valves within the nozzle body, both of which operate with reciprocating action, allow for control of both the resin and pressurized gas flows. The valve associated with the pressurized gas is said to prevent the flow of plastic into the various gas passages. The reciprocating-type operation of these valves do not, however, allow for a positive shut-off. Once plastic material is deposited on the mating surfaces of one of reciprocating valves, that valve cannot completely seal, thereby allowing even more plastic material to be deposited thereon. Once such deposits occur, the gas passageway will quickly become blocked requiring costly downtime for cleaning. And due to the complicated design and small passages sizes, removing such blockages can be difficult.
U.S. Pat. No. 4,942,006 Jul. 17, 1990) described an even more complicated nozzle design. Within the nozzle, resin flow is controlled by a reciprocating-type valve and gas flow by a ball-shaped check valve. And U.S. Pat. No. 5,080,570 (Jan. 14, 1991) provides for a complicated nozzle wherein the plastic flow is controlled by a reciprocating-type valve and the gas flow by a reciprocating-type needle valve. Both of these complicated designs are prone to blockage and, when such blockage occurs, are difficult to clean. In addition, these nozzles--like those described above--are generally bulky due to the complicated design and the number of moving parts. Therefore, mold bodies and sprue bushings with essentially unrestricted access around the sprue opening are generally required. Thus, many of the just described nozzles cannot readily be used with existing mold or sprue bushings having relatively restrictive or limited physical access to the sprue opening.
Still more recent attempts have been made to provide simple injection nozzles that allow injection and venting of the pressurized gas through the nozzle. For example, U.S. patent application Ser. No. 07/628,746 (filed Dec. 17, 1990), which is hereby incorporated by reference, provides for a nozzle with a cylindrical barrel mounted axially within the nozzle. The barrel, which contains a separate resin passageway and a separate pressurized gas passageway, is rotatable between a first position and a second position. In the first position, resin can be injected into the mold cavity but the gas flow is blocked. In the second position, gas can be injected (and later vented) but the resin flow is blocked. U.S. patent application Ser. No. 07/714,118 (filed Jun. 12, 1991), which is hereby incorporated by reference, provides for a nozzle with a spherical valve member mounted within the resin flowpath to control both the resin and pressurized gas flows. The spherical valve member, which contains a separate resin passageway and a separate pressurized gas passageway, is rotatable between a first position and a second position. In the first position, resin can be injected into the mold cavity but the gas flow is blocked. In the second position, gas can be injected (and later vented) but the resin flow is blocked. And U.S. patent application Ser. No. 07/714,117 (filed Jun. 12, 1991), which is hereby incorporated by reference, provides a nozzle with a cylindrical valve member mounted perpendicular to the resin flowpath which can also control both the resin and gas flows. The cylindrical valve member, which contains a separate resin passageway and a separate pressurized gas passageway, is also rotatable between a first position and a second position. In the first position, resin can be injected into the mold cavity but the gas flow is blocked. In the second position, gas can be injected (and later vented) but the resin flow is blocked.
Although the nozzle designs described in patent application Ser. Nos. 07/628,746, 07/714,117, and 07/714,118 have proven to be satisfactory for many applications and are a significant improvement over prior art designs, there is still room for improvement. These nozzles still experience significant downtime unless the operator is careful during operation of the injection molding process, especially during the start up and shut down procedures, due to blockage of the various passageways with plastic material and other operational difficulties.
It is desirable, therefore, to develop an even simpler and more reliable injection nozzle for gas-assisted injection molding whereby the gas can be vented through the nozzle. Such a nozzle should allow for almost continuous operation with only minimal operator attention. And such a nozzle should be easy to clean during normal maintenance procedures or should any of the passageways eventually become blocked with plastic. The nozzle of the present invention provides just such a nozzle.